Nickel-base superalloys are extensively employed in high-performance environments. However, the fabrication of current high-strength .gamma.'-strengthened nickel-base superalloys having the best high temperature properties encounter serious problems in attempts at fabrication by forging. These problems relate to the high solvus temperature of the .gamma.' phase, which will have a value very close to the incipient melting temperature of the alloy.
For this reason, direct hot-isostatic pressing (HIP) of powder superalloys has been used extensively to produce large scale critical components for aircraft engines, such as turbine disks. In addition to being able to avoid the forging problems, tne near-net shape processing employed in HIP processing yields cost savings by reducing both the amount of input material required and the machining cost. However, a characteristic of this type of processing is the occurrence of internal defects, such as voids and ceramic formations in the parts formed, because of the inability of the art to produce perfectly clean powder. As a result, the performance of parts prepared in this manner may be impaired, because such defects play a key role in the response of the part material under cyclic stress. While considerable efforts has been expended to improve powder metallurgy (e.g., improvement in the cleanliness of powder processing), the nature and morphology of defects in parts made by powder processing and their role as initiation sites for cracking have never been well characterized. The development of high strength alloy compositions free of the alloy processing difficulties encountered in conventional melting, casting and forging remains an alternative solution, particularly for addressing the problem of fatigue crack growth at service temperatures. The development of the superalloy compositions of this invention focuses on the fatigue property and addresses in particular the time dependence of crack growth.
Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (.sigma.) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity K, which is proportional to .sigma..sqroot.a. Under the fatigue condition, the stress intensity in a fatigue cycle may consist of two components, cyclic and static. The former represents the maximum variation of cyclic stress intensity (.DELTA.K), i.e., the difference between K.sub.max and K.sub.min. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (.DELTA.K) until the static fracture toughness K.sub.IC is reached. Crack growth rate is expressed mathematically as da/dN.varies.(.DELTA.K).sup.n. N represents the number of cycles and n is material dependent. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate. For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys. To add to the complexity of this time-dependence phenomenon, when the temperature is increased above some point, the crack can grow under static stress of some intensity K without any cyclic component being applied (i.e. .DELTA.K=0). The design objective is to make the value of da/dN as small and as free of time-dependency as possible. Components of stress intensity can interact with each other in some temperature range such that crack growth becomes the function of both cyclic and static stress intensities, i.e., both .DELTA.K and K.
It is an object of this invention to prepare as a turbine disk material, a nickel-base superalloy [e.g., for preparing a turbine disk by the cast and wrought (C&W) process] having a composition that will guarantee that the alloy can be hot-forged on a large scale. At the same time, the strength of the alloy at room and at elevated temperatures, as well as the creep properties thereof, should be reasonably comparable to those of powder-processed alloys.
The hot workability of nickel-base superalloys in the conventional forging process depends upon the nature of the microstructure of the alloy both prior to and during forging. The as-cast ingot usually displays dendritic segregation. Large ingots of alloys having high age-hardening element content always develop heavily dendritic segregation and large dendritic spacing. Subsequent to this dendritic segregation, large concentrations of thermally stable carbide as well as other intermetallic segregation form and such formations can have a significant effect on the alloy properties. Thermal homogenization treatments can serve to diffuse such dendritic segregation. However, selection of the homogenization temperature that may be used is limited by the problem of incipient melting. Loss of forgeability and deterioration in mechanical properties are evident when even a slight amount of incipient melting occurs. In most instances, the initial ingot conversion operations begin at temperatures well above the .gamma.' solvus with most of the subsequent work being carried out below the .gamma.' solvus. The result is a fully refined structure. If the alloy exhibits a high .gamma.' solvus, one is forced to employ a relatively high temperature in the forging operation. This will cause coarse microstructure to form, because of the in-process annealing that occurs. Such microstructure has low ductility and is sensitive to quench cracking.
It becomes evident, therefore, that in order to develop a superalloy composition that exhibits good fatigue cracking resistance, unique selections of alloy chemistry and microstructure must be made. As will be shown hereinafter, the chemical compositions of the alloys of this invention have been selected through the application of several unconventional metallurgical considerations that control (1) the volume fraction and chemistry of the precipitation phases, (2) the selection of alloy matrix and (3) the selection of microalloy additions. In order to ensure superior resistance to fatigue crack growth in the resulting alloy, it was also necessary to determine what heat treatment should be employed in combination with the foregoing considerations to develop the proper microstructure.
Certain relationships and terminology will be utilized herein to describe this invention. The approximate conversions of weight percent to atomic percent for nickel-base superalloys of the precipitation hardening elements such as aluminum, titanium, tantalum and niobium. are set forth as follows:
Aluminum (wt%).times.2.1=Aluminum (at%)
Titanium (wt%).times.1.2=Titanium (at%)
Niobium (wt%).times.0.66=Niobium (at%)
Tantalum (wt%).times.0.33=Tantalum (at%)
In respect to nickel the term "balance essentially" is used to include, in addition to nickel in the balance of the alloy, small amounts of impurities and incidental elements, which in character and/or amount do not adversely affect the advantageous aspects of the alloy.
More detailed characteristics of the phase chemistry of .gamma.' are given in "Phase Chemistries in Precipitation-Strengthening Superalloy" by E. L. Hall, Y. M. Kouh, and K. M. Chang [Proceedings of 41st. Annual Meeting of Electron Microscopy Society of America, August 1983 (p. 248)].
The following U.S. patents disclose various nickel-base alloy compositions: U.S. Pat. Nos. 2,570,193; 2,621,122; 3,046,108; 3,061,426; 3,151,981; 3,166,412; 3,322,534; 3,343,950; 3,575,734; 3,576,681, 4,207,098 and 4,336,312. The aforementioned patents are representative of the many alloying situations reported to data in which many of the same elements are combined to achieve distinctly different functional relationships between the elements such that phases providing the alloy system with different physical and mechanical characteristics are formed. Nevertheless, despite the large amount of data available concerning the nickel-base alloys, it is still not possible for workers in the art to predict with any degree of accuracy the physical and mechanical properties that will be displayed by certain concentrations of known elements used in combination to form such alloys even though such combination may fall within broad, generalized teachings in the art, particularly when the alloys are processed using heat treatments different from those previously employed.
The objectives for forgeable nickel-base superalloys of this invention are three-fold: (1) to minimize the time dependence of fatigue cracking resistance, (2) to secure (a) values for strength at room and elevated temperatures and (b) creep properties that are reasonably comparable to those of powder-processed alloys, and (3) to reduce or obviate the processing difficulties encountered heretofore.